Celestron C90

C90 is as high quality it can be for 90 mm aperture small telescope. C90 is very compact, and versatile. Having an effective focal length of 1000 mm at f/11, it can be directly mounted on a full frame 35 mm SLR via a T-mount adapter for 20X magnification. To disassemble this telescope, the tripod mount is first removed, and there is a Phillips screw that limits the front focusing barrel. The focusing barrel could then be removed to test the main mirror via an interferometer (below). The spherical mirror on this telescope shows better than 1/10 wave surface accuracy. C90 is perfect for viewing the craters on the moon, the ring of Saturn, and moons of Jupiter.

The focus mechanism

The front focusing ring in C90 is the most convenient technique Celestron has devised in focusing. It very much looks like a Japanese made telephoto lens. The two screws on the base of the telescope (above, right) are responsible provide the rotational stops for near, and far end of focusing rotation for the front barrel.

Primary Mirror Testing with an Interferometer

The interferometer (below) is equipped with interchangeable focusing objectives that focus the interferometer beam to a point, and it starts to diverge again. With the f/10 objective, the beam covers the entire mirror surface. By adjusting the interferometer's height, tilt adjustment, and direction of its beam propagation, the focal point of the primary mirror on the telescope coincides with the focal point of the f/10 objective. At this precise alignment, the interferometer sees the telescope's wavefront as flat, and compares it with its own reference flat for surface accuracy.